Lapping method and tool



T. G. LEWIS LAPPING METHOD AND TOOL Filed May 22, 1950 INVENTOR.

THO/WAS G. LEWIS A TTORNE Y.

through successively finer 1 stages, the

Patented Oct. 20, 1953 UNITED STATES RATENT OFFICE LAPPING METHOD AND TOOL Thomas G. Lewis, Wilmington, DeL, assignor to E. L du. Pont. de Nemours. & Compan Wilmington, Del.,a corporation of Delaware Application May '22, 1950, Serial No. 163,562

.4 Claims.

product will conform to very precise size standards. The lapping of surfaces from their original state to the final finish is a progressive operation, involving. the use. of a series of abrasives ranging from relatively coarse at the beginning sizes to the end. The results secured depend; upon a number of factors,

such as the properties of the abrasive employed,

the pressure with which the abrasive is forced against the work, the pattern vof movement preserved in the contact of. the work with the abrasive'particles, and other considerations.

Hitherto, the theory has been that good lapping action depends upon the retention of abrasive against the work by th lapping tool. it been the practice of the prior art to utilize as lapping materials substances which have the property of retaining more abrasive on their working surfaces than the workv which it is desired to lap. Where work having a relatively rough initial suriace is to be processed, .it is advantageous to accomplish the lapping in two first being a rough lapping operation employing atool capable of removing material at a rate when usedin conjunction with coarse abrasive, while the second is a-fine lapping operawherem a finer abrasive is utilized for the removal of any scratchesor other imperfections remaining from the first stage. To secure high :retentionof abrasive in the lapping. tools used in the roughlappingstage, has been thegpractice of the prior art to employ tools fabricated from materials. softer than the work being. processed, so that at least a substantial proportion of the abrasive particles will embed themselves in the toolsurface and wih thereupon .ahrade the surface of the work :away in somewhat the: same manner as-a conventional lathe tool removes material by its action. .Customarily, thefme lapping operation has been conducted with materials which are much soiter than those .em-

-ployed for rough lapping, but againwit has been considered desirable,

or at least not disadvan- 2; .tageous, to employ substances which act to retain the abrasive preferentially to the work, and thereby maintain a relatively high concentration of the wearing substance at the tool face.

In both cases, the abrasive particles are bound to a considerable degree to the working toolsurface, and the same general portions of the retained particles continue to contact the work during operations, so that wear is largely localized in these areas, while th remainder of the. abrasive grains do not carry their share of the burden. It will be understood that. many of the abrasive particles are not immediately retained by the lapping tool and may pass through the working area in a relatively free manner a number of times as the tool andthe Work-are moved withrespect to one another. The result is that the..particles have a widely varying freedom of movement which tends to-distribute the abrading burden non-uniformly, causing scratches or other blemishes in the work. The situation is aggravated by the dislodgement-of particles: from the work surface during the course of lapping, which particles. appear to agglomerate into larger masses on the fac .oilthe lapping tool, where they eventually mar the surface of the work in thesame manner as would an exceptionally large abrasive particle if. it entered upon the field currently worked by a liner sizeof abrasive. The eiiect of agglomerates is usually notas long-sustained as would be the --case with a.- coars abrasive particle but, nevertheless, the damage is accomplished whenever a deep scratch results, even though the length of the blemish may not be great. Under these circumstances, some areas of the work require intensive special lapping treatment toremove local defects before they acquire the same q l ty of surface as the balance of the work,

which requires time and the services of specially trained personnel.

1 have new found that an improved lapping action is -,o-bt ained by conducting operations so that the movement of the abrasive particles is c p bl -o yieldin ahish degree or reproducibility in operation. I

Another object of this invention is to provide lapping compositions having a low retentivity for abrasive materials, which give improved results in both rough and fine lapping operations.

These and other objects of this invention are attained by employing a contacting pressure, a random movement of the abrasive with respect to the work and a lapping tool les retentive of abrasive than the work being processed, so that rolling movement of the abrasive particles over the surface of the work being treated is favored.

In order to illustrate one preferred arrangement for applying the invention, reference is made to the accompanying drawing in which- Figure 1 is a perspective view of one design of a lapping head provided with a backing material embodying this invention, while Figure 2 is a vertical section taken along line 22 of Fig ure 1.

The lap comprises a hard surfacing material which may be tool steel, or glass of equivalent hardness, or a softer material for subsequent surfacing such as tetrafiuoroethylene, and 12 designated a backing material having a plasticity subject to cold fiow under the conditions of operation, which may be bitumen. The head I I, is adapted to support the lapping surface and may have its forward face arcuately shaped or recessed to conform to the profile of the work,

'as contemplated and illustrated in companion applications Ser. Nos. 163,560 and 163,561, referred to elsewhere herein. The backing material may be interposed in this recess between the lap l0 and the head H as indicated at l2. The backing material may be supported directly, or encased in a fibrous bag as indicated in application Ser. No. 163,560, and may be clamped to head II by straps I l, secured by bolts iii. The head. It may be operated in various ways, but is particularly suitable for operation by the lapping 11 machine of my above application Ser. No. 163,561, in which a powered means is adapted to engage self-aligning sleeve l6, while the head II is supported in oscillating position by lugs I? as shown in said co-pending application.

Since it is desirable to first remove the surface material of the work at a relatively high rate, which is decreased as the finish is improved, I

prefer to conduct my lapping in two distinct stages, utilizing different laps in each. In the initial rough lapping operation, a very hard tool, such as one fabricated from tool steel, glass or material of the same or greater hardness is preferred. In the fine lapping operation I prefer to employ a softer lap, such as one fabricated 'from polymerized tetrafiuoroethylene, for example, the surface of which appears to resist attachment of the abrasive thereto to an extraordinary degree. For best results the surfaces of the laps should conform as closely as possible to the surface of the work, so that an even pressure will be transmitted to the abrasive suspension at all points over the full working area. In the case of the flexible polytetrafluoroethylene lap, close conformation to the work may be achieved by backing the lap with a material having a plasticity at room or slightly higher temperatures such that the material will cold flow to an extent sufficient to take a surface the reverse of that of 1 the work under the lapping pressures employed,

this backing being described in detail in my copending application, Ser. No. 163,560, filed May 22, 1950.

As hereinabove described, lapping is accomplished by the use of abrasives of progressively i decreasing particle size. Any of the commonly used abrasives may be utilized in the methods of this invention, including aluminum oxide, silicon carbide, diamond dust and other materials. The inherent crushing strengths of these abrasives appear to have little effect on the abrading action secured, the particles continuing to remove materials from the work surface uniformly, even after being fractured. The sizes of abrasives employed will depend to some extent upon the surface condition of the work as received and the resistance to abrasion possessed by the work material. In the rough lapping of copper electroplates, case hardened steel, 18-8 type stainless steels and nickel cladding, I have found that good results may be achieved by the employment of the following sizes of aluminum oxide abrasive (e. g. the Norton Companys 38 series material) in sequence, identified by grit number, using a tool steel lap of a Rockwell C hardness of about 60, a lap pressure of about 10 lbs/sq. inch and a sperm oil vehicle, the concentration of the abrasive in the oil being maintained at about 8 0z./gal.: 180, 240, 320, 400, 500, 700, and 1000. The resulting finish has a general quality such that an average surface defect measures about 6 to 8 microinches, as determined by the conventional root-mean-square method with a profilometer. At this point a diamond dust abrasive may be substituted for the aluminum oxide, and operation continued with the same lap, operating pressure and oil vehicle, with a concentration of about 1 carat of abrasive per 50 cc. of oil, through a range of grit sizes from about 3200 to about 6400, when a general surface quality of 2 to 3 microinches R. M. S. is obtained. A polytetrafiuoroethylene lap may now be substituted for the tool steel and the fine lapping operation performed using a 1 carat/50 cc. dispersion of 6400 grit diamond dust in olive oil, until a very high quality finish, approximating A to microinches R. M. S., is obtained.

The liquid vehicle employed in the method of this invention should be one capable of simultaneously wetting the work, the lap and the "abrasive. Animal and vegetable oils of the type represented by sperm oil and olive oil, respectively, have proved entirely satisfactory, uniform dispersion of the abrasive therein being facilitated by the addition of up to about 1% by volume of one of the commercially available dispersing agents, such as, for example, Antarox B and E200, manufactured by the General Aniline & Film Corporation. The viscosity of the liquid vehicle should be low enough to preserve the mobility of movement of the abrasive particles therein. In practice, sperm oil of a viscosity below about 100 seconds on the Saybolt scale has given good results as a vehicle for aluminum' oxide and silicon carbide abrasives in the particle size range from about through about 1000 grit. Diamond dust suspended in olive oil appeared to have a considerably lower settling rate than when sperm oil was employed as the vehicle, thus olive oil is preferred as the vehicle for fine lapping operations, and the limiting upper viscosity on the Saybolt scale was determined corresponds to about two-thirds the diametrical dimension of, the average particle. The film thickness can be most conveniently regulated by applying the diamond. dust-oil suspension to the work sparingly, then observing the extent at which abrading proceeds, and increasing or decreasing the rate of application accordingly.

The surface texture of the polytetrafluoroethylene lap is an important factor in obtaining best results by the method of this invention, particularly when relatively long laps of large surface area. are employed at pressures in excess of about lbs/sq. in. It is preferred to break the surface by slitting it to a depth of 0.040" to 0.060 in a uniform square pattern, each unit of which measures about 0.015" to about 0.125" on a side, one diagonal of the squares being parallel to the width dimension of the work when the lap and, the work are at rest before relative motion is initiated. In this connection, it will be understood that the depth of the. slits is limited so that the lap is not weakened appreciably by the surface roughening imparted by the slitting, and a polytetrafiuoroethylene lap A thick may be treated as described without sacrificing much of its useful strength. The primary purpose of the slit lines is to relieve pressure built up in the oil film, particularly with very fine abrasives such as diamond dust suspended in olive oil, so that the thickness of the film will not exceed a value about two-thirds that of the average abrasive particle diameter, at which point abrading action is considerably reduced or even entirely halted. An additional efiect of surface slitting is that the flexibility of the lap is thereby somewhat improved, so that the formation of hard spots resulting from nonhomogeneities in the lap material is inhibited.

The movement of the lapin contact with the Work should be of a random nature such that the pattern described is non-repetitive to a high degree. In the hand lapping of the test pieces described in the following examples the lap was moved in a circular or elliptical manner,;with the work being continuously turned with respect to the lap so that the successive paths described by the abrasive particles continually crossed one another. Powered equipment operating to produce a periodic spiral pattern repetitive only" at relatively long time intervals has proved equally satisfactory. In both instances best results are obtained at operating pressures below about 18 v lbs/ sq. in., the highest quality finishes being'ob- *tained in the lapping" of most materials at pressures in the neighborhood of 10'lbs./sq. in., at which level the free rolling movement of the --the work in the manner hereina'bove described for random movement, the work specimens being measuring approximately fiat 'metalpieces v A contacting pressure of about 1-0 lbs./sq.in-. Was'maintai'ned throughout.

Example. I

A tool steel sample having a hardness of 60 Rockwell 6 was processed with a tool steel lap "having a hardness of '60 Rockwell C, using the following successive grit sizes of'aluminumoxide suspended in a sperm oil vehicle: 180, 320., 500, and 1000. The time of treatment ranged from about 30 minutes for the 180 size grit to about 10 minutes for the 1000 size grit, the total time consumed in the rough lapping operation being about minutes. At the 'endof this period the average quality of the finish measured about '6 microinches R. M. S. An intermediate lapping operation using the same tool steel piece em ployed for the rough lapping was then conducted with 3000 and 6000 grit sizes of diamond dust in sequence suspended in olive oil as the vehicle, the time required being about 15 minutes for the 3000 grade and 15 minutes for the 6000 grade abrasive. An average finish of quality measuring about 2 microinches R. M. S. was thereby secured. The sample was finished with a surf ace-slitted polytetrafluoroethylene lap employin 3000 and 6000-grit diamond dust, respectively, suspended in olive oil, for a time of "10 minutes for the 3000 size and 20 minutes for the 6000 size, the final surface quality of the specimen measuring about microinch R. M.-S.

Example 2 A mild S. A. E. 1020 steel sample having a hardness of 35- Rockwell B was processed with :a tool steel lap having a hardness of 60 Rockwell C, using the following successive grit sizes. of aluminum oxide suspended in a sperm oil vehicle: 180, 320, 500 and 1000. The time of treatment ranged from about 2.0 minutes for the size grit to about 10 minutes for the 1000- size grit, the total time consumed in the rough lapping operation being about 60 nunutes. At the-conclusion of this treatment, the average quality of the finish secured was about 8 microinches M. S. The specimen was then fine lapped. with a surface-slitted polytetrafluoroethylene tool using 3000 grit size and 6.000 grit size diam-0nd dust suspendedin an olive oil vehicle in sequence, the time required for each of these operations being about 20' minutes. The quality'of the final finish measured about /2 microinch R. Mas.

Example 3' A commercial bronze sample having ahardness; of about 180 Rockwell B-was; rough lapped with a piece of tool steel of -60 Rockwell C hardness, employing 320;,- 500 and 1=G.00qg:r it sizeswof aluminum. oxide suspended in sperm oil sequence, the duration of; each stepbeing about 15 minutes-rand: total time for .thearough lapping aggregating 45 minutes.

imprevement to. 1 microinchB. S.

Example 4:

An aluminum samplewith'a hardhessof about '70 on the Rockwell 13 scalewas rough lapped with a piece of tool steel havinga Rockwell 6" hardness of 60 for a period of 15 -mi-nutes with 500 grit size, and 15 minutes with 1 000" g-rit size, aluminum oxide suspended in sperm oil, to -give a surfacequality measuring'ab'out 10 mi'croinches R. S. The'sample-was then finelf 'pnedwith a piece of surface-slitted' polytetrafiiioreethylene for 1 5- minutes with 3000 grit and '20 minutes 75 with=6000 grit diamond dust suspended: in olive 'minutes, respectively. quality measured about 10 microinches R. M. 8.,

.oil, yielding a final surface quality of about 1 microinch R. M. S.

Example 5 A specimen of electroplated copper havin a hardness of about 70 on the Rockwell B scale was rough lapped with a piece of tool steel having a Rockwell C hardness of 60, using 320 and 500 size grits of aluminum oxide abrasive suspended in sperm oil in sequence for periods of 15 minutes each. The surface finish quality obtained meas-- ured about microinches R. M. S., after which the work was subjected to a fine lapping treatment employing a surface-slitted polytetrafiuoroethylene lap and 3000 and 6000 grit sizes of diamond dust suspended in olive oil in turn. After a treatment of minutes duration with the 3000 size diamond abrasive and minutes .with the 6000 size material, the quality of the surface finish obtained measured about microinch R. M. S.

Example 6 A specimen of electroplated nickel having a hardness of about 85 on the Rockwell B scale was given a preliminary lapping treatment with a piece of tool steel having a Rockwell C hardness of 60, employing 320 and 500 size grits of aluminum oxide suspended in sperm oil as the abrasives for periods of minutes and 15 The resulting surface whereupon a fine lapping operation was conducted with a piece of surface-slitted polytetrafluoroethylene and 3000 and (E000 grit sizes of diamond dust suspended in olive oil. After minutes lapping with the 3000 grit size and 30 minutes with the 6000 grit size materials, a surface finishquality measurin about microinch R. M. S. was attained.

Progressive microscopic examination of the specimens prepared according to the procedures described in the foregoing examples indicated that the surface had a uniformly indented appearance, somewhat resembling that obtained by shot blasting, except that the indentations were,

of course, much smaller in size. The surface carried few of the straight scratches which characterize work lapped by the abrasive stones or cast iron laps heretofore employed in the art. An additional advantage was the practical elimination of the graininess or pigskinlike texture from'steel and nickel surfaces displaying microscopic variations in hardness, which has previously not proved amenable to removal by lapping.

Thesurface appearance of materials treated according to this invention supports the conclusion that the beneficial effects are produced by the rolling movement of the abrasive particles. A high degree of uniformity of finish results after sustained operation with a given size 7 of abrasive, and the quality improves as lapping is continued, indicating that all of the particles are progressively worn away to about the same extent. While all of the' factors hereinabove described are important for the maintenance of the abrasive in rolling movement, the low retentivity of the laps is believed especially important to success.

- material and observing the new finish secured. The lapping tools may be freed of the coarse ma- 'terial by-simply washing thoroughly with a solvent for the oil vehicle, such as a chlorinated hydrocarbon, for example, whereupon no scratches will develop with the fine abrasive which would indicate retention of any of the larger size material previously used. Lapping tools of low retentivity possess an additional advantage in that it has been observed that metal particles from the work do not tend to agglomerate to masses which attain sufiicient size to deface the Work, either because the tool does not provide a surface for build up, or because the masses are eroded away by the wearing action of the abrasive particles as they come into contact one with another in the course of rolling.

This method of lapping and lap therefor may form an element of my invention disclosed in my copending application Ser. No. 163,561, filed May 22, 1950.

As many apparent and widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended patent claims.

I claim as my invention:

1. The method of lapping a metal article comprising applying to said article a particulate abrasive suspended in an oil vehicle, forcing said abrasive against said article with a material having a Rockwell C hardness of 60 or higher at a pressure below about 18 lbs/sq. in. while maintaining relative random movement between said article and said material, and progressively decreasing the size of said suspended abrasive from .coarse to fine as the surface finish of said article ,is improved, then forcing successively smaller sizes of finer abrasive suspended in oil against said article with a mass of polytetrafiuoroethylene at a pressure below about 1.8 lbs/sq. in. while maintaining relative random movement between said article and said mass of polytetrafiuoroethylene to produce a high quality final finish.

2. An abrading lap comprising a body of substantially homogeneous polytetrafluoroethylene having on its working surface a parallelogram pattern of intersecting slit lines of a depth of 0.040 to 0.060, half the thickness of said body, with parallel slit lines separated by distances ranging from about .0015" to about 0.125", and capable of surfacbut not exceeding about oneing metal articles with particulate abrasives suspended in liquid vehicles.

3. The method of lapping a casting wheel of substantial diameter and having a metallic surface, with a lapping tool having a concave-curved surface conforming to the surface of and maintained in parallel and horizontal relation with the casting wheel, comprising lapping the cylin- I parallel, horizontal position, to produce a uniform final surface finish for film casting.

4. The method of lapping a casting wheel of substantial diameter and having a metallic surface, with a lapping tool having a concave-curved surface conforming to the surface of and maintained in parallel and horizontal relation with the casting wheel, comprising lapping the cylindrical work piece in two stages, first with a lap of glass having a hardness equal to tool steel, and. then with a lap of solid tetrafluoroethylene, backed by a material capable of slight cold flow at normal room temperature under the conditions of operation and under a pressure not exceeding about 18 pounds per sq. in., repeating the treatment with a suspension of lapping material of decreasing particle sizes, and maintaining random movement of the surfaces with respect to each other to freely roll the abrasive over the opposed surfaces of the work while maintaining the surfaces in substantially parallel, horizontal position, to produce a uniform final surface finish for film casting.

THOMAS G. LEWIS.

References Cited in the file of this patent UNITED STATES PATENTS Number 5 Re. 14,751 Re. 22,770 257,612 336,582 1,318,300 1,343,522 1,356,330 1,682,420 1,979,689 2,230,654 2,353,171 2,412,306

Name Date Brockbank Nov. 18, 1919 Sjorgren July 2, 1946 Renshaw May 9, 1882 Morris Feb. 23, 1886 Robertson et al Oct. 7, 1919 Robertson et al June 15, 192.0 Allen Oct. 19, 1920 Rolf Aug. 28, 1928 Hoke Nov. 6, 1934 Plunkett Feb. 4, 1941 MacC'onnell July 11, 1944 Stoll Dec. 10, 1946 OTHER REFERENCES Laps and Lapping, pages 969-972, inclusive, 20 from Machinerys Handbook, 5th edition, 1918,

N. Y., Industrial Press. 

1. THE METHOD OF LAPPING A METAL ARTICLE COMPRISING APPLYING TO SAID ARTICLE A PARTICULATE ABRASIVE SUSPENDED IN AN OIL VEHICLE, FORCING SAID ABRASIVE AGAINST SAID ARTICLE WITH A MATERIAL HAVING A ROCKWELL C HARDNESS OF 60 OR HIGHER AT PRESSURE BELOW ABOUT 18 LBS./SQ. IN. WHILE MAINTAINING RELATIVE RANDOM MOVEMENT BETWEEN SAID ARTICLE AND SAID MATERIAL, AND PROGRESSIVELY DECREASING THE SIZE OF SAID SUSPENDED ABRASIVE FROM COARSE TO FINE AS THE SURFACE FINISH OF SAID ARTICLE 